Low-temperature fractionation process



' Aug. 29, 1967 I 1.. J.HVIZDOS ET AL 3,333,061

LOW-TEMPERATURE FRACTIONATION PROCESS Filed Aug. 12, 1964 MiW A TTOR NEY GASEOUS N2 PRODUCT United States Patent 3,338,061 LOW-TEMPERATUREFRACTIONATION PROCESS Leonard J. Hvizdos, Allentown, and John A. Pryor,Emmaus, Pa., assignors to Air Products and Chemicals,

Inc., a corporation of Delaware Filed Aug. 12, 1964, Ser. No. 389,093Claims. (Cl. 62-13) This invention relates to improvements in theseparation of normally gaseous mixtures, more particularly, to thelow-temperature separation of air into its components, especiallyrelatively pure oxygen and high purity nitrogen.

In the low-temperature fractionation of air, a conventional methodcomprises the utilization of two columns, generally in superimposedrelationship, wherein a high pressure column is used to produce a crudeoxygenenriched mixture as bottoms and a nitrogen-enriched mixture asoverhead, the bottoms fraction and overhead fraction being passed asfeed and reflux, respectively, to alow pressure column.

In order to produce high purity nitrogen, a system is described in US.Patent No. 2,762,208 wherein the high pressure column is provided withadditional plates, thereby forming a nitrogen-enriching section.Additionally, at the top of said column, the pressure is maintained at ahigh level in the column, and of most importance, substantially all ofthe overhead nitrogen vapor is condensed and then refluxed to the top ofthe same column, the patent indicating that it is preferred for onlyabout 3% of the liquefied high purity nitrogen to be removed from theprocess. As the patent further states, the condensed nitrogen isreturned to the top of the high pressure rectifying zone in order toreflux the rectification of air into a high purity nitrogen eflluentwithout extensive gas-liquid contact and controls. In other words, thepatent operates at almost total reflux, thereby reducing the requirednumber of theoretical plates to almost the theoretical minimum. On theother hand, the utilization of almost total reflux involves anadditionalconsumption of energy as compared to normal operation, as wellas a column of inordinately larger diameter to accommodate therelatively large volume of vapor throughput, and also equipment forpumping the reflux back to the top of the column.

An object of the present invention, therefore, is to provide an improvedprocess for the production of oxygen, together with nitrogen of highpurity.

Another object of this invention is to provide a process wherein theair, after being cooled in switching heat exchangers or the like, isfurther cooled by heat exchange with gaseous oxygen product, gaseousnitrogen from the high pressure column, and gaseous nitrogen from thelow pressure column.

Still another object is to provide a novel process for the separation ofa gaseous mixture into its components.

Upon further'study of the specification and the appended claims, otherobjects and advantages of this invention will become apparent.

To achieve the objects of this invention, there is provided a processfor the low-temperature separation of a normally gaseous mixture into atleast one enriched high boiling fraction and at least one enriched lowboiling fraction, which process comprises the steps of:

(a) Fractionating the gaseous mixture in a main high pressure separationzone to obtain an overhead fraction and a bottoms fraction;

(b) Passing said bottoms fraction as feed to a main low pressureseparation zone;

(c) Passing a portion of said overhead fraction to an auxiliary highpressure fractionation zone positioned away from the main high pressureseparation zone and tractionating said overhead [fraction to produce anenriched low boiling overhead; and

(d) Passing at least a portion of said enriched low boiling overhead inindirect heat exchange relationship with said at least one enriched highboiling fraction, said fraction being withdrawn as liquid bottoms fromthe main low pressure separation zone, to condense said at least aportion of said enriched low boiling overhead, and to vaporize said atleast one enriched high boiling fraction.

By passing only a portion of the overhead fraction. to an auxiliary highpressure fractionation zone, positioned away from the main high pressurefractionation zone, it is possible to produce enriched low boilingfraction economically. Furthermore, by condensing the enriched lowboiling overhead with enriched high boiling fraction, both gaseousenriched high boiling fraction and liquid enriched low boiling fractionare obtained.

In some cases, moreover, it is desirable to produce gaseous enriched lowboiling fraction which can be utilized, as such, in a nearby plant.Under such circumstances, it is desirable to produce as much gaseousenriched low boiling fraction as possible, and according to thisinvention, it is beneficial to produce the gaseous enriched low boilingfraction by employing another fractionation zone, which is auxiliary tothe main low pressure fractionation zone. In this auxiliary low pressurezone, the resultant enriched liquid low boiling overhead from step (d)is employed as reflux liquid in direct countercurrent contact with aportion of overhead vapor from the main low pressure separation zonewherein an enriched overhead gaseous low boiling fraction is obtained.The bottoms fraction, comprising a less rich low boiling fraction, isthen returned to the main low pressure column as reflux. By utilizingthis modification, it is possible to produce more gaseous enriched lowboiling product than would be possible by merely utilizing the gaseousenriched low boiling product produced in the auxiliary high pressurezone.

As specifically applied to the production of high purity oxygen andnitrogen, a system is provided wherein there is no attempt to obtainhighly pure nitrogen in the high pressure column, but instead, impurenitrogen vapor is withdrawn from the top of the high pressure column,and is subjected to additional fractionation in an auxiliary highpressure column .for the production of highly pure nitrogen, i.e., nogreater than about 50 ppm. 0 This pure nitrogen is then advantageouslycondensed by heat exchange with liquid oxygen product which is withdrawnfrom the bottom of the low pressure column. A portion of the resultantpure liquid nitrogen is employed as reflux for the auxiliary highpressure column. The remaining portion can then be used as product, orwhen an additional quantity of gaseous nitrogen product is desired, thepure liquid can be beneficially employed as reflux in a low pressurezone. This process is particularly advantageous for the production ofgaseous oxygen which is used in steel refining operations.

Another feature of this invention comprises a novel and eflicient heatexchange technique wherein a portion of the incoming gaseous mixture isheat exchanged prior to fractionation with three streams, as follows:(1) gaseous enriched high boiling fraction, (2) gaseous portion of saidoverhead fraction from the high pressure separation zone, and (3) agaseous portion of the overhead fraction from the low pressureseparation zone.

It is believed that the preceding description of the invention issufiicient to enable one skilled in the art to use it to its fullestextent. For purposes of illustration, however, reference is directed tothe accompanying drawing which is a diagrammatic representation of apreferred embodiment of the present invention.

Refer-ring to the drawing in detail, 147,000 lbs/hr. of air enter thesystem at 10, are compressed in compressor 11 to about 87 p.s.i.a., andare thereupon sent through conduit 12 to the switching heat exchanger13. The function of the switching heat exchanger is to cool the air toabout 277 F., as well as to precipitate congealable impurities, such asCO When the streams are reversed, waste nit-rogen is passed through theconduit formerly containing the compressed air in order to sublime andremove the impurities therefrom. It is to be noted that there are othermethods of removing the CO from the air, such as by a caustic wash, orby the utilization of regenerators instead of switching heat exchangers.

In the embodiment illustrated in the drawing, the air is passed throughconduit 14, whereas the Waste nitrogen isv passed countercurrentlythereto in conduit 15. The air is in heat exchange with gaseous nitrogenproduct in conduit 1-6 and gaseous oxygen in conduit 17. Additionally,some cooling is obtained from high pressure nitrogen in line 18 which isexpanded in expander 19 to form additional cold waste nitrogen.

After the cooled and purified air leaves the switching heat exchanger,it is passed by conduit 20 to the bottom part of fractionating column21. This bottom part 21H, hereinafter designated as the high pressurefractionation or separation zone or column, operates at about 85.0p.s.i.a. The air is introduced below the plates in this high pressurecolumn, but out of contact with the body of liquid in the bottom of thecolumn. A minor portion of the air (about 2,400 lbs/hr.) is withdrawnfrom the bottom of the high pressure zone and is introduced by conduit22 into three heat exchangers in parallel, namely, heat exchangers 23,24 and 25 wherein the air is cooled by oxygen product in exchanger 23,by high pressure nitro gen in exchanger 24, and by waste, low pressurenitrogen in exchanger 25. This cooled minor air portion at 283 F. isthen passed through conduit 26 and combined with the bottoms liquid fromthe high pressure fractionatin-g zone 21H. This bottoms liquid amountingto about 73,000 1bs./hr. at 279 F. is obtained via a conduit 27, and thecombined portions from conduits 26 and 27 are passed via conduit 28 asfeed to the low pressure column, but first through a heat exchangerwhich will be described in detail infra.

Referring now to the operation of the high pressure column, it isoperated at as low a pressure as possible in order to separate the airinto a crude nitrogen-fraction containing 16% oxygen as an impurity,preferably about 2.8%. In this connection, it is important to emphasizethe fact that while it is technically feasible to increase the pressureand add additional plates to the top of the high pressure column toobtain a purer nitrogen fraction, this has been found to be economicallyundesirable in accordance with the teachings of this invention.

Vapor, at 286 F., from above the plates in the high pressure column iswithdrawn in line 29 and divided at 30 into two portions. One portion ofabout 15,500 lbs/hr. is passed to heat exchanger 24 in heat exchangewith air, as previously described.

The other portion of the gaseous high pressure nitrogen stream, about40,000 lbs./hr., is introduced into the bottom of auxiliary highpressure fractionating column 32 via conduit 31. In this auxiliarycolumn, the nitrogen is purified to a purity of less than 50 p.p.m. O toless than p.p.m. 0 depending upon the desired rate of production of pureN The overhead vapor emanating from column 32 (about 40,000 lbs/hr. atabout 286 F. and 85 p.s.i.a.) is passed through conduit 33, and then isoptionally divided into two portions at point 34. One of said portions,in the illustrated embodiment about 5,500 lbs./hr., is passed throughline 35 into heat exchanger 36 in heat exchange with closed circuitnitrogen which is employed in a refrigeration cycle. The resultantcondensed high purity nitrogen is withdrawn from heat exchanger 36 andis passed through line 37 to combine with additional high puritynitrogen at point 38. The other portion of the high purity nitrogen ispassed into heat exchanger 39 and is condensed therein in heat exchangewith high purity oxygen from the bottom of the low pressure column 21Lwhich is superimposed above the high pressure column, in fractionatingcolumn 21. The main high and low pressure columns are connected throughvaporizer-condenser 76, wherein all the re-boiler vapor for the main lowpressure column is provided by the condensation of a portion of overheadvapor in the main high pressure column. In this way, there is limitedreboil in 21L, with pure gaseous oxygen being produced outside thecolumn, and not recycled as reboil vapor. The condensed high puritynitrogen from heat exchanger 39 is withdrawn and passed through line 40to combine at point 38 With the fluid in conduit 37. It is to be furtherunderstood that the amount of pure N which is condensed by closedcircuit nitrogen refrigeration is 0-100% of the contents of line 33,depending on the desired quantity of liquid oxygen as product. Aparticularly preferred embodiment of the invention, however, is tocondense some of the pure N; with pure liquid oxygen from the main lowpressure fractionating column, thereby effecting substantial savings inenergy and equipment.

The combined portions in 37 and 40 then pass through line 41 and aresplit at point 42 wherein about 35,000 lbs/hr. are passed through line43 as reflux for column 32. This amount of reflux yields a nitrogenpurity of less than about 5 p.p.m. oxygen at a production rate of about100,000 s.c.f./hr. of nitrogen. For higher production rates andcorrespondingly less purity, e.-g. less than about 50 p.p.m. at 200,000s.c.f./-hr., the reflux ratio is correspondingly reduced.

The other portion of the purse liquid nitrogen is then passed throughline 44 through heat exchanger 45 in heat exchanger relationship withhigh purity gaseous nitrogen. The cooled liquid nitrogen at about 82p.s.i.a. and 312 F. is then passed through line 46 to pressure reducingvalve 47. After its pressure is lowered to about 19 p.s.i.a., formingsome gas phase, the resultant mixture at 316 F. is passed into phaseseparator 48.

The liquid which is collected at the bottom of the phase separator canthen be deployed according to market conditions. It can be passed toliquid nitrogen storage in line 49, or instead, according to thepreferred illustrated embodiment, it can be passed as reflux in line 50to auxiliary low pressure column 51, Obviously, it is also possible todivide the liquid at point 52 so that some of it goes to liquid nitrogenstorage and some of it is employed as reflux. This provides the processwith substantial flexibility for responding to market conditions. Thepurpose of low pressure auxiliary column 51 is to provide additionalpure nitrogen over and above the output in line 44 of the auxiliary highpressure column 32. This is accomplished by fractionating overheadnitrogen emanating from the low pressure column, said nitrogen beingintroduced into the auxiliary low pressure column via conduit 53. Theamount of available nitrogen in line 53, about 7,200 lbs./hr., islimited, in the illustrated embodiment, by the necessary amount ofscavenging gas in line 59. In a different modification wherein the airis cleaned by a different procedure, such as caustic scrubbing, thislimitation would not be present.

The impure bottoms liquid (about 4,800 lbs./hr. at

about 4% O in auixilary low pressure column 51 is then passed as refluxto the top of the main low pressure column via line 54. The pure gaseousnitrogen from column 51 (about 7,000 lbs/hr.) and phase separator 48(about -200 lbs/hr.) is then passed through line 55 to heat exchanger 45where it is warmed by heat exchange relationship with the pure liquidnitrogen; and then the resultant warm pure gaseous nitrogen is passedthrough line 56 into conduit 16 of the switching heat exchanger 13 whereit is further warmed. The resultant warmed gaseous pure nitrogen productis withdrawn in line 57 and can either be stored, or continuouslyconsumed, in accordance with consumer demand.

Returning now to below the main high pressure column 21H, the combinedportions in line 28 (about 75,000 lbs/hr. at 280 F. and about 85p.i.s.a.) are passed to heat exchanger 58 wherein they are heatexchanged with overhead impure gaseous nitrogen from the low pres surecolumn, said impure nitrogen being withdrawn from conduit 59. After thisheat exchange step, the combined portions are then introduced into thefeed point of the low pressure column via conduit 60.

In order to obtain additional reflux for the main low pressure column,impure, liquid nitrogen (about 13,000 lbs./hr. at 287 F. and about 85p.s.i.a.) is withdrawn from the high pressure column at line 61 and isalso passed through heat exchanger 58 in heat exchange with with theoverhead gaseous nitrogen stream from the low pressure column. Thecooled impure liquid nitrogen is then withdrawn from heat exchanger 58in line 62 and is passed through pressure reducing valve 63 so that itcan then be set as reflux at about 19 p.s.i.a. and 316 F. to the top ofthe low pressure column 21L via line 64.

'For even more reflux, the impure liquid nitrogen which is less pure(about 2.8% oxygen) than the nitrogen present in line 61, is withdrawnfrom the bottom of the auxiliary high pressure column 32. It is alsopassed through heat exchanger 58 via conduit 65 and then passed throughpressure reducing valve 66 and combined with the liquid nitrogen fromthe top of the high pressure column at point 67, both streams goingthrough conduit 64 to the top of the low pressure column.

According to the preferred embodiment of this invention, oxygen isproduced in the low pressure column with a minimum purity of 99.5%. Thisliquid oxygen which is withdrawn from the low pressure column in line 68(about 32,000 lbs/hr. at 22 p.s.i.a. and 290 F.) is first passed throughhydrocarbon adsorber 75, then passed through heat exchanger 39 where itis vaporized, thereby condensing a portion of the overhead nitrogen fromauxiliary high pressure column 32. The oxygen is not completelyvaporized in one pass through the heat exchanger 39. so it is withdrawnfrom said heat exchanger and passed into phase separator 70 via line 69.Liquid oxygen is collected at the bottom of the phase separator and aportion can be sent to liquid oxygen storage, if desired, or it can berecycled via line 71 through heat exchanger 39, thereby completing athermal syphon cycle. In the preferred illustrated embodiment, about3,100 lbs./hr. are recovered as lox.

The gaseous oxygen product is withdrawn from the top of phase separator70 and is passed through heat exchanger 23 via conduit 72. The warmedoxygen stream is then passed from heat exchanger 23 into heat exchanger13, specifically into conduit 17 where it is further warmed, and iswithdrawn as the warmed gaseous oxygen product. It is of additionalinterest to note that the gaseous oxygen is not returned to the main lowpressure column 21L, and that a pressure leg is suflicient driving forceto enable the liquid oxygen to flow through the hydrocarbon adsorber.

Lastly, the impure nitrogen overhead from the top of the main lowpressure column, after having been warmed in heat exchanger 58, isthereupon further warmed in heat exchanger 25 via conduit 73. The warmedimpure gaseous nitrogen is then withdrawn from heat exchanger 25 and ispassed, via conduit 74, into the switching heat exchanger 13,specifically, in the illustrated embodiment, into conduit 15 where itsublimes and purges the impurities. The contaminated nitrogen is thenWithdrawn and passed to waste.

Whereas the preferred embodiment of the invention is related to theseparation of air, it is to be understood that this invention is notlimited thereto. As one skilled in the art will appreciate, theprinciples of this invention can be applied to the separation of othergaseous mixtures, particularly cryogenic fluids, such as helium-con- 5taining gases and mixtures of hydrocarbons.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and intended to be, within the full range of equivalence ofthe following claims.

What is claimed is:

1. A process for the low-temperature separation of a vnormally gaseousmixture into at least one enriched high boiling fraction and at leastone enriched low boiling fraction, which process comprises the steps of:

a (a) fractionating the gaseous mixture in a main high pressureseparation zone to obtain an overhead fraction and a bottoms fraction;

(b) passing said bottoms fraction as feed to a main low pressureseparation zone to obtain overhead vapor and an enriched high boilingbottoms fraction;

(c) passing a portion of said overhead fraction to an auxiliary highpressure fractionation zone positioned away from the main high pressureseparation zone and fractionating said overhead fraction to produce anenriched low boiling overhead; and

(d) passing at least a portion of said enriched low boiling overhead inheat exchange relationship with enriched high boiling bottoms fractionwithdrawn from the main low pressure separation zone, to condense saidenriched low boiling overhead, thereby producing liquefied enriched lowboiling overhead, and to vaporize at least a portion of the enrichedhigh boiling bottoms fraction,

. 2. The process of claim 1, further comprising the steps preceding step(a) of:

(1) cooling and cleaning the gaseous mixture in a switching heatexchange zone,

(2) passing the cooled and cleansed gaseous mixture to the main highpressure separation zone,

(3) Withdrawing a minor portion of the cooled and cleansed gaseousmixture from the main high pressure separation zone, and

(4) passing said minor portion. in indirect heat exchange relationshipwith:

(A) the gaseous enriched high boiling fraction,

(B) a gaseous portion of said overhead fraction fromthe main highpressure separation zone,

and

(C) a gaseous portion of said overhead fraction from the main lowpressure separation zone.

3. The process of claim 1, further comprising the step of returning aportion of the liquefied enriched liquid low boiling overhead as refluxto said auxiliary high pressure fractionation zone.

4. Process of claim 1 comprising the further step of withdrawing asproduct the enriched high boiling bottoms fraction following the step(d).

5. The process of claim 1, comprising the further step of passing aportion of the liquefied enriched liquid low boiling overhead to anauxiliary low pressure fractionation zone positioned away from said mainlow pressure separation zone, and employing liquefied enriched lowboiling point overhead as reflux in direct countercurrent contact with aportion of the overhead vapor from said main low pressure separationzone to produce an enriched gaseous low boiling overhead fraction and aless rich liquid low boiling bottoms fraction.

6. The process of claim 5, further comprising the step of returning aportion of the liquefied enriched liquid low boiling overhead as refluxto said auxiliary high pressure fractionation zone.

7. I11 a low-temperature process for the separation of air into oxygenand nitrogen, which process comprises the steps of cooling and cleaningthe air, fractionating the cooled and cleansed air in a main highpressure separation zone to obtain a bottoms fraction enriched in oxygenand an overhead fraction enriched in nitrogen, and passing said bottomsfraction as feed to a main low pressure separation zone to provideoverhead vapor and liquid oxygen bottoms product, the improvement whichcomprises:

(a) fractionating a portion of said overhead fraction in an auxiliaryhigh pressure fractionation zone positioned away from the main highpressure separation zone, said fractionating being sufficient to form anoverhead stream of purified nitrogen and a less pure bottoms stream,

(b) reducing the pressure of the less pure bottoms stream, and

(c) passing said less pure bottoms stream under reduced pressure asreflux to said main low pressure separation zone.

8. The process of claim 7, comprising a further step of (d) passing aportion of said overhead stream of purified nitrogen in indirect heatexchange relationship with liquid oxygen bottoms product withdrawn fromthe main low pressure separation zone, whereby said portion of theoverhead stream of purified nitrogen is condensed, and the liquid oxygenis at least partially made gaseous.

9. The process of claim 8, further comprising the step of passing saidliquid oxygen bottoms product through a hydrocarbon absorption zone,before said liquid oxygen bottoms product is heat exchanged with saidoverhead stream of purified nitrogen. 1

10. The process of claim 8, further comprising the step of returning aportion of the condensed purified nitrogen as reflux to said auxiliaryhigh pressure fractionation zone.

11. The process of claim 8, further comprising the step of (e') passinga part of the condensed purified nitrogen stream, as reflux, to anauxiliary low pressure fractionation zone positioned away from said mainlow pressure separation zone, in direct countercurrent contact withoverhead vapor withdrawn from said main low pressure searation zone, toproduce high purity nitrogen gas.

12. The process of claim 11 wherein only a portion of the overhead vaporfrom said main low pressure separation zone is fractionated in theauxiliary low pressure fractionation zone.

, 13. The process of claim 11, further comprising the steps precedingstep (a) of:

(1) passing the cooled and cleansed air to the bottom of the main highpressure separation zone,

(2) withdrawing a minor portion of the cooled and cleansed air from thebottom of the main high pressure separation zone, and

(3) passing said minor portion in indirect heat exchange relationshipwith (A) a gaseous oxygen product,

(B) a portion of said overhead fraction enriched in nitrogen, from themain high pressure separation zone, and

(C) a second portion of overhead vapor from said main low pressureseparation zone.

14. In a low-temperature process for the separation of air into oxygen.and nitrogen, which process comprises the steps of cooling and cleaningthe air, fractionating the cooled and cleansed air in a main highpressure separation zone to obtain a bottoms fraction enriched in oxygenand an overhead fraction enriched in nitrogen, and passing said bottomsfraction as feed to a main low pressure separation zone to obtain liquidoxygen product as bottoms of the main low pressure separation zone, theimprovement which comprises:

(a) fractionating a portion of said overhead fraction in an auxiliaryhigh pressure fractionation zone, said fractionating being sufiicient toform an overhead stream of purified nitrogen and a less pure bottomsstream, and

(b) passing a portion of said overhead stream of purified nitrogen inindirect heat exchange relationship with liquid oxygen product bottomswithdrawn from the main low pressure separation zone, whereby saidportion of the overhead stream of purified nitrogen is condensed, andthe liquid oxygen is made gaseous.

15. The process of claim 14 wherein the oxygen after step (b) iswithdrawn as product, and wherein all reboil vapor in said main lowpressure separation zone is obtained as a result of heat transferredfrom condensation of a portion of said overhead fraction enriched innitrogen obtained in said main high pressure separation zone, saidcondensation being conducted in a condensingvaporizing zonecommunicating between the main high and low pressure separation zones.

References Cited UNITED STATES PATENTS 3,062,016 11/ 1962 Dennis et al62-22 3,113,854 12/1963 Bernstein 62-29 X 3,127,260 3/ 1964 Smith 62-29X 3,216,206 11/ 196-5 Kessler 6213 3,260,056 7/ 1966 Becker 62-13FOREIGN PATENTS 136,027 1/ 1950 Australia.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,338,061 August 29, 1967 Leonard J. Hvizdos et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below Column 4, line 37, for "purse" read pure line 39, for"exchanger" read exchange line 50, for "51," read 51. line 68, for"auixilary" read auxiliary column 5, line 8, for "p.i.s.a." readp.s.i.a. line 16, for "impure," read impure line 20, strike out "with";line 24, for "set" read sent column 7, line 32, for

' "absorption" read adsorption line 45, for "searation" read separationSigned and sealed this 5th day of November 1968.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. A PROCESS FOR THE LOW-TEMPERATURE SEPARATION OF A NORMALLY GASEOUSMIXTURE INTO AT LEAST ONE ENRICHED HIGH BOILING FRACTION AND AT LEASTONE ENRICHED LOW BOILING FRACTION, WHICH PROCESS COMPRISES THE STEPS OF:(A) FRACTIONATING THE GASEOUS MIXTURE IN A MAIN HIGH PRESSURE SEPARATIONZONE TO OBTAIN AN OVERHEAD FRACTION AND A BOTTOMS FRACTION; (B) PASSINGSAID BOTTOMS FRACTION AS FEED TO A MAIN LOW PRESSURE SEPARATION ZONE TOOBTAIN OVERHEAD VAPOR AND AN ENRICHED HIGH BOILING BOTTOMS FRACTION; (C)PASSING A PORTION OF SAID OVERHEAD FRACTION TO AN AUXILIARY HIGHPRESSURE FRACTIONATION ZONE POSITIONED AWAY FROM THE MAIN HIGH PRESSURESEPARATION ZONE AND FRACTIONATING SAID OVERHEAD FRACTION TO PRODUCE ANENRICHED LOW BOILING OVERHEAD; AND (D) PASSING AT LEAST A PORTION OFSAID ENRICHED LOW BOILING OVERHEAD IN HEAT EXCHANGE RELATIONSHIP WITHENRICHED HIGH BOILING BOTTOMS FRACTION WITHDRAWN